Mitosis and Meiosis

Metaphase 1

- Like metaphase in mitosis, during metaphase 1 in meiosis the chromosomes also line up on the equator of the cell. In meiosis, chromosomes stay in their homologous pairs. So, instead of 46 individual chromosomes lining up, there are 23 pairs.

Prophase 1

- Synapsis occurs during prophase 1. All the chromosomes have to find their homologous partner and pair up (chromosome 1A from mom has to find chromosome 1B from dad). - When synapsis is complete, all the chromosomes are paired with their partners, so instead of finding 46 replicated chromosomes floating around, we find 23 pairs of replicated chromosomes. - Because each pair consists of 4 chromatids (2 chromatids per chromosome and 2 chromosomes), this pair is known as a tetrad. - All of the other normal events that occur in prophase still happen. The spindle is formed, the chromosomes condense, and the nuclear membrane disintegrates. - After synapsis occurs, an event called crossing over takes place. Basically, this means that like segments on homologous chromosomes are exchanged.

Meiosis II

- The cells we are starting with in meiosis II are haploid only (have 23 chromosomes) - During prophase 2 the spindle forms, nuclear membrane disintegrates, and the DNA condenses (there is no pairing of chromosomes because there is nothing to pair it with because the homologous partners were separated during anaphase 1). - During metaphase 2, the chromosomes line up individually along the equator - During anaphase 2, the centromere splits and the chromatids divide, then the chromatids are called chromosomes - During telophase 2, a nuclear membrane forms around the newly split chromosomes, resulting in 4 genetically unique haploid cells.

Telophase

1. Chromosomes cluster at opposite ends of the cell 2. Supercoiled chromosomes begin to unravel and to return to their pre-cell division condition as long, threadlike strands 3. The nuclear membrane reforms

Metaphase

1. Chromosomes line up single file located on the equator or metaphase plate 2. Centrosomes are at opposite poles of the cell 3. Spindle fibers run from centrosomes to the centromeres of the chromosomes

Prophase

1. Strands of chromosomes begin to condense and become visible 2. The nucleoli disappear 3. Spindle fibers begin to form in the cytoplasm, extending from one centrosome to another. 4. Centrioles begin to migrate to the poles. They form a bunch of fibers called the mitotic spindle. These fibers attach to the chromosomes at their centromeres and help to push and pull them during mitosis. 5. The nuclear membrane begins to disintegrate

Anaphase

1. The centromere that joins each pair chromatids splits in two so that each chromatid separates from its paradise. Then the newly separated chromosomes move to opposite poles of the cell with the help of the spindle fibers.

Meiosis I

1. This stage is also called reduction division 2. Synapsis and crossing over occur. During synapsis, chromosomes pair up precisely with their homologue so that crossing over can occur. Crossing over is the process in which homologous chromatids exchange genetic material. Crossing over is important because it ensures greater variety in the gametes 3. Homologous chromosomes separate. Failure to separate is nondisjunction 4. Chromosomes line up randomly on the equatorial plate and separate independently. This means that how one pair of chromosomes lines up and separates has no effect on how any other pair of chromosomes line up and separate 5. Each resulting gamete is genetically unique

Meiosis

A form of cell division in which cells having the diploid chromosome (2n) produce gametes (sex cells) with the haploid chromosome number (n). Meiosis occurs in two stages (I and II).

Chromatids

After interphase, each chromosome and the duplicate piece of DNA that was just made are held together at their center by a region called a centromere. Chromatids describe each of the individual chromosomes (each chromosome has two chromatids).

Mitosis

Division of nucleus (prophase, metaphase, anaphase, and telophase).

Anaphase 1

During anaphase 1 of meiosis, the centromeres DO NOT divide (individual chromosomes do not split). Instead the homologous pairs (the two chromosomes) separate, with one entire replicated (a pair of chromatids and a centromere) moving to each of the opposite poles of the cell.

Interphase

More than 90 percent of a cells life is spent in this stage. During interphase chromosomes replicate in preparation for cell division. One or two nuclei become visible within the nucleus, and the nuclear membrane remains intact. Interphase is composed of three phases G1, S (when chromosomes replicate), and G2.

Sex-cells

Sperm and ova (gametes) only have only 23 sets of chromosomes. Cells that have two sets of chromosomes are diploid, Cells that have one set of chromosomes are haploid, thus sperm and ova are haploid cells. When the sperm fertilizes the ovum, the cell formed is a diploid zygote.

Telophase 1

Telophase 1 of meiosis is very similar to telophase of mitosis. The two cells finish dividing their cytoplasm (cytokinesis), and nuclear membranes reform around the chromosomes. The two new cells DO NOT have 23 pairs of chromosomes (46 total chromosomes), they have 23 replicated chromosomes (each chromosome is made of two identical chromatids). Because there are no homologous pairs, the cells are considered haploid by telophase 1.

Cytokinesis

The division of the cytoplasm. In animal cells, a cleavage furrow forms down the middle of the cell as the cytoplasm pinches inward and the two daughter cells separate from each other. In plant cells a cell plate forms down the middle of the cell. Daughter cells do not separate from each other. Instead a sticky middle lamela cements adjacent cells together.

Independent assortment of chromosomes

The way that chromosomes line up during metaphase 1 affects that outcome of the genetic information in the gametes that form. This is because genes on non-homologous chromosome pairs are inherited independently of one another. For example, if the haploid number (n) = 4, then 2^n = 16, so there are 16 possible combinations for chromosomes in gametes.If genes are on the same chromosome, they are called linked genes and are inherited together. Crossing over may separate linked genes.